Man, the highest type of living things, differs from the rock, moreover, in that he possesses the power to exercise his will in directing natural forces. Animals and even plants seem to possess a similar power to a smaller degree. The rock on the hillside is pulled downward by gravitation, but can move only if the ground is removed from beneath it by some external force. Man, on the other hand, can walk up or down the hill, with or against the pull of gravity. [Sidenote: Science teaches that all phenomena may be referred to matter and ether in motion.] Modern science refers all phenomena to matter and motion; in other words, to matter and force or energy. In this general sense, matter includes the universal ether, and force includes any or all of the forces known, or that may be known, to man. To illustrate: the electrician develops a current of electricity, which to the scientist is a portion of the universal ether moving in a certain definite manner. When the vibrations of the ether are caused to change, light, or magnetism or chemical affinity may result from the electricity. In every case, matter is in motion. The ear perceives a certain sound. It is produced by the movements of the air. In fact, sounds are carried from place to place by great air waves. The heat of the stove is due to the rapid vibration of the molecules in the iron of the stove, which set up corresponding vibrations in the ether. In nature no exceptions have been found to the great scientific claim that all natural phenomena may be explained by referring them to matter in motion.[A] Variations in the kind of matter and the kind of motion, lead to all the variations found in the universe. [Footnote A: Tyndall, Fragments of Science, I. chaps. I and II.] [Sidenote: Life is a certain form of motion.] By many it has been held that life and its phenomena transcend the ordinary explanations of nature. Yet, those who have learned, by laborious researches, that the fundamental ideas of the universe are only eternal matter, eternal energy and the universe-filling medium, the ether, find it very difficult to conceive of a special force of life, which concerns itself solely with very limited portions of matter, and is wholly distinct from all other natural forces. To the student of science it seems more consistent to believe that life is nothing more than matter in motion; that, therefore, all matter possesses a kind of life; and that the special life possessed by plants, animals and man, is only the highest or most complex motion in the universe. The life of man, according to this view, is essentially different from the life of the rock; yet both are certain forms of the motion of matter, and may be explained ultimately by the same fundamental conceptions of science. Certainly, such an idea is more beautifully simple than that of a special force of life, distinct from all other natural forces.